Machine dishwashing detergents constitute a generally recognized distinct class of detergent compositions. In general, machine dishwashing detergents are mixtures of ingredients whose purpose, in combination, is to breakdown and remove food soils; to inhibit foaming caused by certain food soils; to promote the wetting of wash articles in order to minimize or eliminate visually observable spotting and filming; to remove stains such as might be caused by beverages such as coffee and tea or by vegetable soils such as carotenoid soils; to prevent a buildup of soil films on wash ware surfaces; and to reduce or eliminate tarnishing of flatware. An additional and critical characteristic which the machine dishwashing detergent must possess is the ability to perform all of the above tasks without substantially etching or corroding or otherwise damaging the surface of glasses or dishes. It is particularly critical that the fading and loss of luster from brightly colored decorations on glasses and dishes be prevented.
In conventional institutional and domestic dishwashing formulations, a strongly alkaline solution is produced and is used to wash dishes, glasses, and other cooking and eating utensils. Ordinary tap water can be used in preparing these strongly alkaline cleaning solutions and for rinsing the wash articles subsequent to the cleaning step. However, in European applications, this tap water is often treated (softened) to remove hardness ions such as calcium and magnesium with the result that hard water residues on washware are reduced. Nevertheless, spotting and filming from soil residues and precipitates can remain a problem, especially if the ion exchange unit serving the dishwashing machine is operating inefficiently. This problem can be minimized with a machine dishwashing composition containing a relatively high level of polyphosphate which acts to sequester hardness ions and to aid in soil removal and stabilization. In addition, these detergents usually contain a chlorine bleaching system for stain removal and for an added cleaning boost via oxidation of proteinaceous soils, thus helping to eliminate spotting on glassware.
Although the cleaning performance of these conventional detergent compositions is satisfactory, high phosphate levels, chlorine bleach, and high alkalinity have potential environmental and consumer drawbacks. As a result, an alternative technology was developed to deliver less alkaline products. Similarly, nonphosphated builders are substituted to further improve the environmental profile of the composition. As a consequence of the reduced cleaning efficiency of the modified composition, various detersive enzymes including amylolytic and proteolytic enzymes are included in the detergent composition in order to boost removal of starchy and proteinaceous soils, respectively. Because these enzymes are not compatible with chlorine bleach systems, an oxygen bleaching system can be substituted which can result in a reduction in bleach performance. Often, enzymatic compositions based on oxygen bleaches are formulated with a phosphate builder, in markets where local legislation will allow, to assure good overall performance. An unfortunate weakness in the performance of this alternative technology, both in formulations which are phosphated (i.e., containing inorganic phosphate builder salts) and those which are nonphosphated, is that they are particularly prone to attacking patterned glasses and plates. The striking color of this patterning is often a key reason for the purchase of the article and its rapid fading after a relatively few dishwashing cycles can be particularly noticeable and give rise to an unfavorable rating by the consumer of an otherwise premium performing machine dishwashing product.
It is an object of the present invention to provide compositions, suitable for use in machine dishwashing methods, having a reduced tendency to fade or otherwise corrode brightly patterned plates and glasses while at the same time maintaining good cleaning performance towards soiled articles.
UK Patent Application GB 2 295 625 A and WO 96/17051 disclose compositions for use in machine dishwashing comprising a mixture of disilicate and metasilicate in which the weight ratio of disilicate to metasilicate is from 50:1 to 3:2. The minor proportion of metasilicate is described to reduce glass pattern corrosion.
WO 96/20268 describes a copolymer of an organomineral siliconate, obtained by condensation polymerization of an alkali metal disilicate and an alkali metal siliconate, as an additive in a machine dishwashing formulation for the purpose of reducing weight loss and visible corrosion on glass.
WO 96/20129 discloses an alkali metal silicate partially substituted with calcium, magnesium, strontium or cerium as counterion. This modified silicate, when incorporated into a machine dishwashing composition, is described to reduce the weight loss and visible corrosion of washed glassware. WO 96/12783 describes the inclusion of a crystalline layered silicate of the general formula Na.sub.2 Si.sub.x O.sub.2+1.yH.sub.2 O for preserving the color and luster of patterned glassware during machine dishwashing.
The prior art thus describes the use of specific silicates or modified silicates to avoid dishwashing fading or corrosion. This basis restricts the type of formulation to which these solutions are applicable. In particular, corrosion of patterned glassware can be quite severe with formulations of low alkalinity, where silicates are of limited use because of their low stability.
Recently, the use of transition metal salts, particularly of aluminum, has been described in Angevaare et al., U.S. Ser. No. 08/444,502 for use in mitigating the fading of colors from decorated glassware. The use of aluminum complexes with such as citrate has been described for the same purpose in Angevaare et al., U.S. Ser. No. 08/786,357 and U.S. Patent No. 5,624,892. Formulations containing silica for reducing glass pattern corrosion are described (Alan Tomlinson, LDC.)